So, a larger allaince (Like TEST, GoonSwarm, RUS, etc) wouldn't be able to get a fleet of 150 people to sit on your undock, while maybe 15 guys flew around to cap the nodes?

This change just makes it slightly easier to hold a system if you defend it IF you are fighting against someone of equal size or skill. Larger entities will still be able to hell camp a system while simultaneously capture nodes.

You say that they would "leave their space undefended." Not so. There are still timers and defensive indices to contend with. Even if the aggressing force were to start being attacked, there would still be a timer to deal with. Once you entosis a system, there is a reinforcement timer, which allows a force plenty of time to get home to defend while still achieving their goal.

For example: Group A attacks Group B. Group A has 300 members. Group B has 100. Group A uses 250 members to hellcamp Group B's station, making it impossible to undock with loosing thier entire fleet. The other 50 Group A members move off to capture the nodes and complete the event.

So, what if Group A starts getting their structures reinforced by Group C? That's no problem. A timer will still form on Group A's structures, giving them time to complete capturing the system/station belonging to Group B before they head back.

Literally the only differences between dominion sov (still better than this new mechanic IMHO) is that there will be multiple smaller fleets split off from the larger main fleet, allowing CCP's servers to not collapse under the weight of 400-500 people in a single system. Also supers will now be fairly useless still (could only really bash structures or other supers with them anyway).